即使在onCreateView中初始化后，android片段视图也会引发空指针异常 [英] android fragment view throw null pointer exception even after being initialized in onCreateView

问题描述

这里我有一个片段交互，如果单击回收者视图项，它将数据传递给 MainActivity ，然后 MainActivity 调用 DetailFragment.updateText()更新视图的方法，但是即使在初始化视图之前调用 DetailFragment.updateText()，即使我在 onViewCreated()中执行了此操作，也不会初始化视图那我如何确保在视图初始化后调用它们.*请注意，我通过XML片段标记将 DetailFragment 添加到了 DetailActivity 中，对于 ListFragment 和 MainActivity

here i have a fragment interaction, where if recycler view item get clicked it passed data to MainActivity and then MainActivity call DetailFragment.updateText() method to update it's view but the views are not initialized even though i did that in onViewCreated(), if DetailFragment.updateText() is getting called before views are initialized then how can i make sure they get called after views have been initialized. * note i added the DetailFragment to a DetailActivity through XML fragment tag, and the same for ListFragment and MainActivity

MainActivity

MainActivity

public class MainActivity extends AppCompatActivity implements ListFragment.Listener {

// the method to be called when an item in recycler view is clicked
// so i can pass this data to DetailFragment

@Override
public void listenerMethod(String firstName, String lastName) {
    DetailFragment detailFragment = new DetailFragment();
    detailFragment.updateText(firstName, lastName);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

}

ListFragment

ListFragment

public class ListFragment extends Fragment {

private static final String TAG = "ListFragment";

private RecyclerView recyclerView;
private RecyclerViewAdapter recyclerViewAdapter;

// fragment communication interface
public interface Listener {
    void listenerMethod(String firstName, String lastName);
}

private Listener listener;

@Override
public void onAttach(@NonNull Context context) {
    super.onAttach(context);
    try {
        this.listener = (Listener) context;
    } catch (ClassCastException e) {
        Log.d(TAG, "onAttach: "+ e.getMessage());
    }
}

public ListFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_list, container, false);
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    recyclerView = getView().findViewById(R.id.recyclerview);
    recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));


    // some dummy data to fill the recycler view
    ArrayList<User> users = new ArrayList<>();

    users.add(new User("hiwa", "jalal"));
    users.add(new User("mohammed", "abdullah"));


    recyclerViewAdapter = new RecyclerViewAdapter(users, getActivity(), listener);

    recyclerView.setAdapter(recyclerViewAdapter);
}
}

RecyclerViewAdapter

RecyclerViewAdapter

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder> {

private List<User> userList;
private Context context;
private ListFragment.Listener listener;

public RecyclerViewAdapter(List<User> userList, Context context, ListFragment.Listener listener) {
    this.userList = userList;
    this.context = context;
    this.listener = listener;

}

@NonNull
@Override
public ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.user_row
            , parent, false);
    return new ViewHolder(view);
}

@Override
public void onBindViewHolder(@NonNull ViewHolder holder, int position) {

    User user = userList.get(position);
    holder.tvFirstName.setText(user.getFirstName());
    holder.tvLastName.setText(user.getLastName());


}

@Override
public int getItemCount() {
    return userList.size();
}

public class ViewHolder extends RecyclerView.ViewHolder {

    public TextView tvFirstName;
    public TextView tvLastName;

    public ViewHolder(@NonNull View itemView) {
        super(itemView);
        tvFirstName = itemView.findViewById(R.id.row_first_name);
        tvLastName = itemView.findViewById(R.id.row_last_name);

 itemView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
          User user = userList.get(getAdapterPosition);

            listener.listenerMethod(user.getFirstName(), user.getLastName());
        }
    });


    }
}
}

DetailFragment

DetailFragment

public class DetailFragment extends Fragment {

private TextView tvFirstName;
private TextView tvLastName;

public DetailFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    return inflater.inflate(R.layout.fragment_detail, container, false);
}

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    tvFirstName = view.findViewById(R.id.detail_frag_first_name);
    tvLastName = view.findViewById(R.id.detail_frag_last_name);
}

// update the details fragment views
public void updateText(String firstName, String lastName) {
    tvFirstName.setText(firstName);
    tvLastName.setText(lastName);
}

}

activity_main

activity_main

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <fragment
        android:id="@+id/main_fragment_list"
        android:name="com.example.peacewithfragments.ListFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />


</LinearLayout>

activity_detail

activity_detail

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".DetailsActivity">


    <fragment
        android:id="@+id/detailsActivity_fragment_container"
        android:name="com.example.peacewithfragments.DetailFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

</LinearLayout>

推荐答案

您不能直接从 MainActivity 访问 DetailFragment ，因为它不是 MainActivity的一部分.因此，首先您必须导航到 DetailActivity ，然后访问 DetailFragment .检查以下内容:

You can't directly access DetailFragment from your MainActivity as it's not part of MainActivity. So, first you have to navigate to DetailActivity and then access DetailFragment. Check below:

public class MainActivity extends AppCompatActivity implements ListFragment.Listener {

    private DetailFragment detailFragment;

    @Override
    public void listenerMethod(String firstName, String lastName) {
        if(detailFragment != null) {
            detailFragment.updateText(firstName, lastName);
        } else {
            Intent detailIntent = new Intent(this, DetailsActivity.class);
            detailIntent.putExtra("FirstName", firstName);
            detailIntent.putExtra("LastName", lastName);

            startActivity(detailIntent);
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        View view = findViewById(R.id.tablet_detail_container);
        if (view != null) {
            detailFragment = new DetailFragment();

            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();


            transaction.add(R.id.tablet_detail_container, detailFragment);
            transaction.addToBackStack(null);
            transaction.commit();
        }
    }
}

然后在您的 DetailsActivity 中接受这些额外的内容，并将其传递给 DetailFragment ，如下所示:

Then in your DetailsActivity accept those extra and pass it to DetailFragment like below:

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_detail);

    ...

    String firstName = getIntent().getStringExtra("FirstName");
    String lastName = getIntent().getStringExtra("LastName");

    DetailFragment detailFragment = getSupportFragmentManager().findFragmentById(R.id.detailsActivity_fragment_container);

    if(detailFragment != null)
        detailFragment.updateText(firstName, lastName);
}

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